Computers don't speak English or any other human language. Deep down in the operating system heart, they speak a language comprised of 0s and 1s, or bits. While the first computer programmers wrote their programs in bits, someone determined that it would be easier to have the computer convert human language into bits rather than do the conversion manually. To allow humans to type, a code was developed to convert human letters into 0s and 1s that the computer could understand.
ASCII is a 7-bit character set where each character is defined by 0s and 1s. Every letter in English, and most Romance language (like French or Italian) characters are represented by an ASCII code. For example, the letter A is ASCII code 65. When you write in a text editor, the letters you type are converted to ASCII before they are sent to the computer processor.
There are only around 45 keys on your standard keyboard - with the shift key, that's about 90 characters - because to a computer uppercase and lowercase letters are different. If you want to use characters not found on your keyboard, you need to use special characters. You can use these HTML codes to write characters that are not found on most keyboards, like á or ÿ. Learn to use special characters.
ASCII text is not the best character set. It is missing many many letters, and languages all around the world cannot even be represented by it. For example, you can't write Russian using ASCII. For that, a new character set has been developed called Unicode. Unicode is a 16-bit character set that includes 65,536 individual characters.